4 CARL GOTTFRIED HARTMAN 



He found that cleavage begins almost exactly 5 days after 

 copulation (compare Hill and O'Donoghue's 'post-oestrus' period^ 

 and that birth takes place 7 days and 20 hours later. The 

 author states that all of the 7 to 27 eggs or embryos found in any 

 one animal are practically in the same state of development. No 

 external indication of pregnancy, no changes even in the mam- 

 mary glands, were to be noted until near the time of parturition. 



iiccording to Selenka, fertilization takes place in the upper 

 part of the oviduct and cleavage begins in the uterus, the eggs 

 scattering and remaining for three days floating in the uterine 

 fluid. Maturation was not observed, although sections of ova- 

 ries were made and nine eggs were taken from the Fallopian 

 tube. 



Except for a few unessential details, Selenka correctly described 

 the unsegmented egg of the opossum. His specimens were for 

 the most part shrunken, as indicated by the shape of the ova 

 and the 'perivitelline space' between the zona pellucida and the 

 albumen layer, a condition I have not found in well-fixed normal 

 eggs. 



The first of Selenka's two specimens yielded one 2-celled and 

 one 20-celled uterine egg and nine unfertilized eggs from the 

 oviduct, all badly shrunken and of little value for study. It is 

 interesting to note, however, that the 2-celled egg showed some 

 elimination of yolk and that the 20-celled stage was practically 

 a young blastocyst and contained an included ('entoderm') cell. 



The second specimen furnished eggs in the following stages: 

 one each in the 4-, 8,- 42- and 68-celled stages respectively; 2 

 young blastocysts containing some entoderm; and 8 advanced 

 blastocysts ranging from 1.05 to 1.43 mm., the largest showing 

 a layer of mesoderm in the embryonic area. 



The blastomeres of the 4-celled stage were conical in form and 

 of equal size and structure. They are radially arranged about a 

 cleavage cavity. The 8-celled stage is made up of two super- 

 imposed rings of four cells each. From these two specimens 

 Selenka concluded that the blastomeres of the 4-celled stage 



1 Hill and O'Donoghue ('13), p. 145. 



